The murine teratocarcinoma system is a unique model system for the study of the regulatory mechanisms controlling tumorigenicity, determination, and differentiation. Embryonal carcinoma cells, the stem cells of teratocarcinomas, are tumorigenic and retain the ability to differentiate into derivatives of all three germ layers. F9 is an embryonal carcinoma cell line, which displays a low spontaneous rate of differentiation in vitro. F9 cells can be induced to differentiate by trans-retinoic acid (RA), 5-bromodeoxyuridine (BudR), and N'-N'-dimethylacetamide (DMA). This differentiation is accompanied by a loss of tumorigenicity. The goal of this research is to isolate multiple classes of F9 variants showing an abnormal resonse to one or more of these inducing agents. Genetic and biochemical analyses of such variants should provide insight into the normal regulatory pathway. One variant (5C) was isolated because it fails to undergo normal morphological differentiation in response to RA. 5C cells differentiate normally in response to DMA and BudR. Further characterization of 5C cells revealed that they undergo a partial differentiation in response to RA, but fail to lose their tumorigenicity. Two 5C "revertants" (5C-9 and 5C-11), which display some morphological differentiation in response to RA, have been isolated. Somatic cell hybrids prepared between F9X5C and F9X5C-9 are currently being characterized. The levels of cytoplasmic RA-binding protein in F9, 5C, 5C-9 and 5C-11 cells are currently being determined. (M)